Acoustic chirp signals belong to a class of waveforms known as being suitable for use in pulse compression or matched filter receivers. A good introduction to pulse compression waveforms and matched filters is given in “Introduction to radar Systems, Third Edition, by Merril I. Skolnik, McGraw Hill, 2001, ISBN 0-07-118189-X”.
A component that significantly affects performance of a sodar apparatus is the antenna system used to transmit and receive the acoustic signals including acoustic chirp signals. The antenna system preferably operates in a bistatic configuration. A bistatic configuration denotes that separate transmitter and receiver antennas are used, to enable an associated receiver to receive at the same time as a transmitter transmits, so that longer transmit pulses can be used. This arrangement may avoid a dead zone that would otherwise be present if a monostatic sodar configuration was used. As the acoustic signals or pulses may typically be greater than 100 ms and usually around 1 second in length, the dead zone in a monostatic system may be about 150 m for a 1 second acoustic transmit pulse, which is undesirable.
The level of transmitted acoustic pulse in a bistatic sodar apparatus may be about 130 dBA while sensitivity level of the associated receiver may be as low as 0 dBA, being limited by atmospheric noise. This level of sensitivity may be achieved by using matched filter processing in the receiver. In a bistatic configuration wherein the transmitter and receiver may operate simultaneously on separate antennas, it is desirable that isolation or attenuation between the transmitter and receiver should be in excess of the difference between the level of transmitted acoustic pulse and the level of sensitivity of the receiver (130 dBa), to avoid interference between the transmitted acoustic signal with signals received from the atmosphere. However, the signals received from the atmosphere at a range of a few 10's of meters may typically be about 30 dBa, so that a direct signal from the transmitter at a range of a typical separation distance between transmitter and receiver antennas (about 4 m) should be about 10 dB below the level of the first received signals to avoid interference with the received signals. This may reduce a requirement for isolation or attenuation to around 110 dB (130 dBa−(30 dBa−10 dB)).
FIG. 1 shows the effect of interference on a received signal from a transmitted signal when there is insufficient isolation between the transmitter and receiver, e.g. when the level of isolation is similar to isolation achieved in current monostatic sodar systems (approximately 50 dB). In FIG. 1 interference from the transmitted signal shows up with pulse length of 0.35 seconds as a brighter section below 60 m in both amplitude and vertical wind speed. This interference compromises the data received from the atmosphere below 60 m. Further in FIG. 1, there are vertical pulses of interference from outside the system that show up in the vertical wind speed around time 20:15 between a height of 200 m and 400 m. The signal to noise ratio is only around 10 dB up to 100 m as shown in the shaded bar on the right in FIG. 1. This occurs because the receiver is compromised by direct signals from the transmitter. Moreover the receiver may also pick up strong local noise sources, further compromising receiver sensitivity and signal to noise ratio.
As noted above, isolation or attenuation between the transmitter and receiver should be greater than about 110 dB, being the difference between the level of transmitted acoustic pulse signal and the level of first signals received at the receiver with a margin of 10 dB. As the distance between the transmitter and receiver antennas may be about 4 m in a typical installation, this level of isolation needs to be achieved over a relatively small distance. Another aspect of the isolation required is that it should be applied in a substantially horizontal direction, while isolation or attenuation in a vertical direction, ie. towards the atmosphere, should be substantially 0 dB, to avoid attenuating signals being transmitted or received in a vertical direction.
In a paper “Bradley, S. G., “Use of Coded Waveforms for Sodar Systems” Meterol. Almos. Phys. 71, 15-23 (1999)”, Bradley stated that a “For a sodar the use of a longer pulse is generally precluded because the first range gate would be too distant”. This assumes that the receiver can only be turned on after the transmitted pulse is complete as would be the case for a monostatic system, otherwise the receiver may be overloaded and may suffer interference from a direct transmitted signal. However if a bistatic system is used, the limitation of having a too distant first range gate may be substantially overcome if horizontal isolation between the transmitter and receiver antennas could be increased to a level where the transmitted signal has substantially little or no influence on the receiver, eg., greater than about 110 dB for a bistatic sodar system.
The present invention may provide an improved apparatus for sounding the atmosphere and a method for sounding the atmosphere using the improved apparatus. The apparatus may be arranged such that any acoustic signal that is passed directly from a transmitter to an associated receiver is adequately isolated and/or attenuated.
A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was known or that the information it contains was part of the common general knowledge in Australia or elsewhere as at the priority date of any of the disclosure or claims herein. Such discussion of prior art in this specification is included to explain the context of the present invention in terms of the inventor's knowledge and experience.
Throughout the description and claims of this specification the words “comprise” or “include” and variations of those words, such as “comprises”, “includes” and “comprising” or “including, are not intended to exclude other additives, components, integers or steps.